The vast majority of clinically evident osteoarthritis remains either idiopathic or related to some mechanical aberration in the joint stability or alignment. We propose that the osteoarthritic process is characterized by disturbance in the equilibrium between mechanical demands on the joint and the ability of joint tissues to support or adapt to those demands. Four individual projects are proposed to test this hypothesis. Project One further extends observations on the blunt impact animal model where the patellofemoral joint is loaded noninvasively by an externally applied force. This project will be utilized to study the cartilage damage and repair, the biology of the zone of calcified cartilage, and the response of subchondral bone. Project Two will study the canine spinal facet joints as an osteoarthritic model using dissolution of the intervertebral disc as an initiating agent to produce changes in the facet joints leading to degenerative arthritis. Biomechanical evaluation of forces and motions in the facet joints will be performed in vivo. Project Three will extend the study of excessive joint motion to humans, evaluating patients with anterior cruciate ligament deficient knees and normal subjects using a six-degree-of- freedom electrogoniometer during functional tasks. The possibility that neuromuscular control of the joint may influence the stability of the knee will be extended from Project Three to Project Four where the role of ligament and capsular innervation as an influencing element in joint stability and function will be studied in dogs.